It hardens teeth and prevents cavities, but 60 years after it began, fluoridation is meeting new resistance

Somebody put a dead rat in Curtis Smith's mailbox. Someone else has made anonymous phone calls accusing him of trying to poison his neighbors. And all around the usually placid university town of Bellingham, Wash., activists from a group called Citizens Against Forced Fluoride have planted lawn signs adorned with skull and crossbones. "I had no idea it would get this intense," says Smith, 70, a retired dentist who is leading a Nov. 8 ballot initiative to add fluoride to the local drinking water. "These are very angry people."

Angry indeed: fluoridation to fight tooth decay, a hot-button issue from the 1950s--when...